An optical fiber is in general formed by drawing while heating and melting a transparent glass mass or tube called an optical fiber preform (preform) in an optical fiber-drawing furnace.
FIG. 1 shows an embodiment of an optical fiber-drawing furnace according to the conventional technique. Drawing furnaces as shown in FIG. 1 are disclosed in, for instance, Japanese Patent Application Publication No. 3-24421.
As seen from FIG. 1, a furnace body 11 made of stainless steel is in general provided therein with a heater 13 for heating and melting an optical fiber preform 12. A furnace core tube 14 in which the foregoing preform 12 is inserted through the top thereof is positioned inside the heater 13.
The furnace core tube 14 is in general composed of a carbon material and fixed to the furnace body 11 at both upper and lower ends of the latter. The furnace core tube 14 comprises an upper cylindrical part 14a, a funnel-like part 14b, and a lower cylindrical part 14c. The upper cylindrical part 14a is composed of a cylinder having a diameter slightly greater than that of the preform 12. The lower cylindrical part 14c, has a diameter which is smaller than that of the preform 12 and permits the passage of a drawn optical fiber 15. The funnel-like part 14b, is positioned between the upper and lower cylindrical parts 14a and 14c, and thus serves to connect the upper cylindrical part 14a to the lower cylindrical part 14c. The funnel like part 14b has a shape whose diameter is gradually reduced in a direction from the upper portion to the lower portion thereof along the molten portion at the lower end of the preform 12 which is drawn into the optical fiber 15.
An annular thermal insulating material 16 comprising, for instance, a carbon felt material is disposed between the furnace body 11 and the heater 13 so as to prevent any external diffusion of the heat radiated from the heater 13.
Moreover, an upper cylindrical member 17 which is communicated with the upper end of the furnace core tube 14 is disposed on the top of the furnace body 11. This upper cylindrical member 17 is usually made of stainless steel and the upper opening thereof is covered with a cap member 18. Further the upper cylindrical member 17 is provided with a gas introduction port 17a at an upper portion thereof. On the other hand, a lower cylindrical member 19 is positioned at the bottom of the furnace body 11 and is communicated with the lower end of the furnace core tube 14. The lower cylindrical member 19 is commonly formed from stainless steel and is provided, at the lower end thereof, with an opening 19a through which the drawn optical fiber 15 can pass.
An inert gas 20 such as N.sub.2 and He is supplied to the upper cylindrical member 17 through the gas introduction port 17a. The inert gas 20 establishes an inert gas atmosphere within the furnace core tube 14 and is discharged through the opening 19a of the lower cylindrical member 19. The inert gas 20 serves to protect the furnace core tube 14 and the preform from oxidation and to thus keep the internal space of the furnace core tube 14 clean.
Incidentally, the aforementioned furnace core tube 14 is not a cylinder having a uniform diameter over the entire length thereof, but is provided with, at the lower part thereof, the funnel-like part 14b, and the lower cylindrical part 14c. These funnel-like and lower cylindrical parts 14b, and 14c, are incorporated into the drawing furnace for the purpose of preventing any variation in the outer diameter of the optical fiber 15 during drawing the optical fiber. More specifically, the space formed between the inner wall of the furnace core tube 14 and the preform 12 is almost identical to that formed between the inner wall of the furnace core tube and the optical fiber 15 drawn therefrom, along the fiber-drawing direction when core tube is provided with a funnel-like part 14b. This arrangement ensures the inhibition of any turbulent current of the inert gas within the space, as will be apparent from the disclosures of the prior art such as Japanese Patent Application Publication No. 3-24421 listed above.
In the conventional drawing furnace as shown in FIG. 1, however, the cross sectional area of the foregoing space is gradually decreased at the narrowed down portion, i.e., the funnel-like part of the furnace core tube 14 and, therefore, the flow rate of the gas downwardly flowing through the furnace core tube is increased at the narrowed down portion. For this reason, the gas flow rate near the molten portion of the preform is higher than that observed when a straight furnace core tube free of such a narrowed down portion is used. As a result, the molten portion of the preform would undergo change in its shape greater than that observed when such a straight furnace core tube free of such a narrowed down portion is used, thereby causing variations in the diameter of the optical fiber drawn from the preform.
In addition, Japanese Utility Model Application Publication No. 63-127947 discloses a method for drawing an optical fiber which makes use of a drawing furnace comprising a furnace core tube provided with a funnel-like part. According to this method, an inert gas is introduced into the furnace through an outlet for the drawn optical fiber positioned at the lower part thereof. In this method, however, the tip of the molten portion of the preform and the optical fiber immediately after drawing would come in contact with a gas atmosphere having non-uniform temperature distribution due to insufficient heating. As a result, the drawn optical fiber is non-uniformly solidified and thus the resulting optical fiber is liable to have variations in its diameter.
As has been discussed above, the problems of the occurrence of a turbulent current in a gas flow and non-uniform temperature distribution in the drawing furnace cannot satisfactorily be solved by simply forming the lower portion of a furnace core tube into a funnel-like shape, since this arrangement does not sufficiently prevent the variations in the outer diameter of the resulting optical fiber.
On the other hand, Japanese Utility Model Application Publication No. 3-32502 and Japanese Patent Application Laying-open No. 59-88336 each discloses a drawing furnace as shown in FIG. 2. In the drawing furnace shown in FIG. 2, all the members, which have functions identical to those of the members of the drawing furnace shown in FIG. 1, bear the same reference numerals and the details thereof are herein omitted to avoid duplication. The drawing furnace is provided with a first furnace core tube 21 which is a cylinder having the same diameter over the entire length and the upper and lower ends of the furnace core tube 21 are fixed to a furnace body 11, respectively. A lower cylindrical member 22 is disposed on the lower end of the furnace body 11 so as to be in communication with the lower end of the core tube 21. An opening 22a is formed at the lower end of the lower cylindrical member 22 and a drawn optical fiber 15 can pass therethrough. Moreover, a gas discharge port 22b is formed on the lower periphery of the lower cylindrical member 22. In addition, a second furnace core tube 23 having a diameter smaller than that of the first core tube 21 is positioned below the core tube 21 and within the lower cylindrical member 22. The second furnace core tube 23 made of, for instance, a carbon material is communicated to the opening 22a of the lower cylindrical member 22 and has a cylindrical shape which permits the passage of the drawn optical fiber 15 therethrough. The lower end of this second furnace core tube 23 is adhered to and supported by the peripheral portion of the opening 22a of the lower cylindrical member 22.
However, the second furnace core tube 23 is a straight cylinder and covers only the drawn optical fiber 15. Moreover, the second furnace core tube 23 simply covers the portion of the molten preform having a reduced diameter approximately identical to that of the optical fiber 15 even if the upper end of the second furnace core tube covers the lower end of the molten portion of the preform 12.
When an optical fiber 15 is produced by drawing an optical fiber preform 12 in such a drawing furnace, the flow of an inert gas 20 supplied through a gas introduction port 17a is disturbed near the molten portion of the preform 12. In particular, the gas flow is, in the vicinity of the upper end of the second furnace core tube 23, divided into a gas flow 20a which runs through the interior of the second tube 23 and is then discharged through the opening 22a and a gas flow 20b which passes through the exterior of the second tube 23 and is then discharged through the gas discharge port 20b. Accordingly, the gas flow is markedly disturbed at that portion. The gas flow thus disturbed comes in contact with the lower part of the molten portion of the preform 12, i.e., the gas molecules collide with the molten portion of the preform 12. This causes vibrations of the molten portion and in turn the resulting optical fiber 15 drawn therefrom has variations in its diameter. Moreover, the heat transfer efficiency between the gas flow and the molten portion of the preform is greatly influenced by such a turbulent current in the gas flow and this also becomes a cause of variations in the diameter of the resulting optical fiber. Furthermore, since the gas flow is disturbed at the upper portion of the second furnace core tube 23, a stable gas flow is not established within the second furnace core tube 23 and this also becomes an additional cause of variations in the diameter of the resulting optical fiber 15.